Without limiting the scope of the invention, its background is described in connection with neutron detectors.
Detection of neutrons is an exceedingly specific indicator of the presence of fissile materials. As such, neutron detectors are deployed at ports of entry throughout the US for the purpose of detecting and interdicting the movement of an illicit special nuclear material (SNM) or an actual nuclear device [1]. Neutron detectors are also an indispensable tool in geothermal and well-logging for the determination of the formation properties, including the porosity and water (and/or H2) content [2]. Similarly, neutron detectors are also useful for planetary missions for remotely sensing the water (and/or H2) content in the shallow subsurface and/or to determine the surface compositions of planetary bodies [3]. When a fast neutron emitted from a neutron generator strikes a hydrogen nucleus of equal mass, which is present in pore water/oil, it thermalizes. Modern neutron logging tools commonly count thermal and epithermal neutrons by employing pressurized 23He (He-3) gas tube detectors. To the first order, the thermal neutron count is inversely proportional to the hydrogen content (or the porosity) of the rocks. The most widely deployed neutron detectors are helium-3 gas detectors. This is because He-3 has a very high thermal neutron capture cross-section of 5330 barn [4]. However, being a gas, He-3 detectors are inherently bulky. Moreover, its scarcity has had an extreme effect on its price in recent days. Other shortcomings of He-3 detectors are the need of high pressurization (up to 20 atm for 2.25-inch diameter tubes), high voltage application (>1000 V) and slow response speed (˜milliseconds). These attributes prohibit flexibilities in detector design and form factors and also increase measurements/exploration/logging time and costs. Additionally, He-3 gas detectors are most appropriate for operation below 175° C. For well logging, the trend is moving into deep and slim wells where temperatures easily exceed 250° C. For geothermal logging, the environmental conditions are even more extreme where temperatures can be as high as 500° C. Therefore, neutron detectors with enhanced capabilities of operating in extreme environments of high temperatures/mechanical vibration/shock are highly desirable. Solid-state thermal neutron detectors have recently been rapidly developed [5-22] for their obvious advantages including independence from 3He gas, compactness, and low voltage operation. Until this date, the most effective solid-state detector approach has been the micro-structured semiconductor neutron detector (MSND), which has been reported extensively in recent years [5-16]. This type of indirect conversion detector is composed of micro-structured Si filled with either 10B or 6LiF. The detection efficiency depends upon microstructure design, material choice, and depth of the reacting material. The most efficient micro-structured semiconductor based thermal neutron detector that has ever been reported consists of a 10B filled Si microstructure with an efficiency of 48.5% [8, 9]. On the other hand, stacked 6LiF filled Si detectors [6, 7, 11-13] with a certified detection efficiency of 30% have already been commercialized. The theoretical and actual attained detection efficiencies of these existing solid-state neutron detectors are limited by the intrinsic material properties and device architectures employed.
In recent years, single crystalline hexagonal boron nitride (h-BN) wide bandgap semiconductor has emerged as an attractive material for neutron detector applications [23-31]. This is due to the fact that single crystal h-BN films (or epilayers) can be synthesized by epitaxial growth techniques such as metal organic chemical vapor deposition (MOCVD) [23-30] and that the thermal neutron capture cross-section of Boron-10 (10B) isotope is quite high (σ˜3840 barns=3.84×10−21 cm2) [4]. Because it is composed of low atomic number elements, B(5) and N(7), h-BN's interaction with gamma photons is extremely low, which gives rise to an excellent gamma to neutron discrimination ratio below 10−6 [26, 28].
The element B exists as two main isotopes, 10B and 11B in a natural abundance of approximately 20% and 80% respectively [4]. It is only the isotope 10B that can interact with neutrons. FIG. 1 is a plot of neutron capture cross sections as functions of the kinetic energy of neutrons for He-3 (green upper plot), B-10 (orange middle plot), and Li-6 (purple lower plot). FIG. 1 shows that 10B (orange middle plot) has a large capture cross section (σ) of about 3840 barn (σ=3.84×10−21 cm2) for thermal neutrons (neutrons with an energy=25 meV), which is only slightly smaller than a value of σ˜5330 barns for He-3 gas atoms (green upper plot) [4]. [adopted from MIT OpenCourseWare—https://ocw.mit.edu/courses/nuclear-engineering/22-106-neutron-interactions-and-applications-spring-2010/lecture-notes/MIT22_106S10_lec07.pdf. (Slide 27)]. However, as a semiconductor, the density of atoms which can interact with thermal neutrons in 100% 10B-enriched BN (10BN) is N(10B)=5.5×1022/cm3, which is about 550 times higher than that in He-3 gas pressurized at 4 atm. This provides an absorption coefficient for thermal neutrons in 10BN of α=Nσ=5.5×1022×3.84×10−21=211.2 cm−1 and an absorption length of λ=α−1=47.3 μm [28-30]. This thickness is negligibly small compared to the dimensions of He-3 gas detectors which typically have diameters in inches.
Earlier h-BN neutron detectors, such as those disclosed in U.S. Pat. No. 9,093,581 which is hereby incorporated by reference in its entirety, were based on a metal-semiconductor-metal (MSM) architecture with micro-strip interdigital fingers fabricated on h-BN epilayers of several microns in thickness [24-27]. The photolithography technique was used to pattern the interdigital fingers on the surface of h-BN epilayers. Pattern transfer was accomplished using inductively-coupled plasma (ICP) dry etching. The patterns were etched all the way to the sapphire substrate. Metal contacts were deposited by e-beam evaporation. The detection efficiencies of these devices were limited to a few percent at the best since this type of MSM device architecture involves dry etching and is limited to the fabrication of very thin h-BN detectors. Moreover, a fraction of the detection area must be removed by dry etching in the MSM detectors, which sacrifices the overall detection sensitivity. Furthermore, dry etching also induces surface damages, which increases surface recombination and reduces the charge collection efficiency. Therefore, these MSM detectors are only suitable for initial conceptual demonstration. Accordingly, a need remains in the art for solid-state neutron detectors.